The present invention relates to an electrical power-steering device and, in particular, to a fixing structure for a coil assembly for torque detection that is used in an electrical power-steering device.
A device known as an electrical power-steering device for a vehicle takes the rotary output of an electric motor, which is to become an auxiliary steering torque, steps it down by means of a gear train, then transfers it to an output shaft of a steering mechanism to steer the vehicle.
One form of such an electrical power-steering device uses a torsion bar that is connected to both an input shaft that is linked to the steering wheel and an output shaft that is linked to the steering mechanism, with the configuration being such that the steering torque is detected thereby. More specifically, since the torsion bar deforms in accordance with the steering torque applied to the steering wheel, the steering torque can be detected accurately by detecting the magnitude of that deformation. If the steering torque can be detected accurately, it becomes possible to output a suitable auxiliary steering torque.
In known examples of this type of detection device, such as those disclosed in U.S. Pat. No. 5,578,767, U.S. Pat. No. 5,739,616, and U.S. Pat. No. 5,796,014, the impedance of a coil provided within the housing is made to vary in accordance with the applied steering torque, and the steering torque is detected on the basis thereof.
Incidentally, in order to accurately measure the amount of distortion of the torsion bar in such a detection device, it is necessary to fix the coil with respect to the housing in such a manner that it does not move in the axial direction of the output shaft. To overcome that problem, the electrical power-steering device disclosed in Japanese Utility Model Laid-Open Application No. Hei 4-38540, for example, is configured such that a disc spring 34 is disposed between a coil yoke 19B and a snap ring (or something similar, although hereinafter "snap ring" is used) 33 that is fitted into an inner radial surface of a housing 25, and the coil yoke 19B is pre-loaded and fixed by the elastic force of the disc spring 34, as shown in FIG. 4.
However, if the configuration is such that this snap ring 33 is disposed between the coil yoke 19B and a bearing 26, the coil yoke 19B and the bearing are inevitably at some distance apart, which has the disadvantage of increasing the length in the axial direction of the device. It is also necessary to provide an installation groove for the snap ring 33 on the inner peripheral surface of the housing 25, but this has disadvantages in that the number of machining steps is increased and the workability during assembly is bad because the snap ring 33 has to be engaged while the disc spring 34 is being compressed.
In addition, since the snap ring 33 must be compressed diametrically while it is being fitted into the installation groove, part of the circumference thereof has to be cut away. Therefore, the cutaway portion cannot maintain a sealing effect, even when the disc spring is placed into contact with the inner surface of the snap ring 33. In other words, since this cutaway allows the intrusion of foreign bodies from the bearing side, there is a danger that, if such foreign bodies should adhere to the circuit board disposed on the outer peripheral side of the coil yoke, they could have adverse effects on the circuit board. Therefore, in such an electrical power-steering device of the prior art, some sort of means should be provided to prevent the intrusion of foreign bodies, which will inevitably increase the cost.